1. Field of the Invention
The present invention relates to a laser resonator and a laser beam converter for generating a laser beam with high power and high beam quality used for a laser processing machine, etc. In particular, the present invention relates to a laser resonator which emits a high-power laser beam by being constructed so that a laser beam with a single high-order transverse mode is emitted, and to a technique for converting a laser beam with a high-order transverse mode into a laser beam with high beam quality.
2. Description of the Related Art
In recent years, processing with a laser beam, that is, laser materials processing, has been utilized. Laser materials processing is carried out by converging a laser beam with an optical element and irradiating the converged laser beam to a portion to be processed. An optical lens and a mirror are mainly used as the aforesaid optical element for convergence. A diffractive optical element which is constructed so as to exhibit a lens effect by forming a diffraction grating on the surface of a glass substrate may be used as the optical element. A binary optics element, which is a kind of diffractive optical element, can be used for a high power laser beam. Also, there has been utilized laser materials processing carried out such that a laser beam emitted from a laser resonator is converged onto an end face of a core portion of an optical fiber without directly converging the laser beam on a portion to be processed, and is guided into an optical fiber, and subsequently, a laser beam emitted from an end facet of the optical fiber is converged on the portion to be processed.
In most cases of the laser materials processing, a converged laser beam is desired to have a single spot intensity distribution. For some materials processings, if the converged laser beam is divided into plural spots, the laser beam is irradiated onto portions other than portions to be processed. This causes disadvantages that a desired processing can not be achieved, and that the energy of laser beam is dispersed. For this reason, the laser beam energy is not irradiated onto the portion to be processed with high density; causing a problem that processing efficiency is poor. Also, if the laser beam is divided into plural spots when it is guided into the optical fiber, there exists a spot which is not guided to the core portion of the optical fiber, causing a problem that the energy of laser beam is not effectively used. Moreover, the laser beam is converged with use of a spherical lens and the binary optics element which is constructed so as to exhibit the same lens effect as the spherical lens by forming the diffraction grating into a concentric circle shape. In such a case, since the intensity distribution of the converged laser beam is the same as that of the laser beam incident upon the spherical lens and the binary optics element, a laser resonator which produces a laser beam having a single spot intensity distribution is desired as a light source.
A gas laser such as a CO.sub.2 laser or a solid-state laser such as Nd:YAG laser is used as a light source for laser materials processing. As is well known, in the gas or solid-state laser, since there is a transverse mode, laser oscillation takes place in various transverse modes, and the transverse mode in which oscillation takes place varies depending upon environmental conditions. For instance, in a TEM.sub.00 mode, a laser beam has a Gaussian distribution in its electric field amplitude, and also, has a Gaussian distribution in its intensity, namely, the laser beam is formed into a single spot. On the contrary, in a high-order transverse mode than the TEM.sub.00, the laser beam is divided into plural spots. In other words, the laser beam with a high-order transverse mode has a beam quality worse than the TEM.sub.00 -mode laser beam.
In the case where an output power of the laser resonator is low, the laser resonator can stably oscillate a laser beam in the TEM.sub.00 mode. However, if the output power of the laser resonator is made high, oscillation takes place in a high-order transverse mode than the TEM.sub.00. Thus, it is desirable that a light source for laser processing has as high output power as possible. In particular, taking workability, cost or efficiency into consideration, it is desirable that as high power as possible can be obtained with the identical laser resonator. But, as seen from the above description, if the output of the laser resonator is made high, the laser beam becomes a high-order transverse mode than the TEM.sub.00 ; for this reason, there has arisen a problem that the output power from the laser resonator can not be made high as desired while maintaining the beam quality of the output to be high. Thus, there has been desired development of a technique capable of providing a high-power laser beam having high beam quality.